This invention relates to optical components. Optical components, for example lenses, prisms and beam deflectors, are normally made from one or more pieces of refractive, optically transparent material shaped such that the variation in optical path length across the wavefront transmitted by the component of light incident on the component produces the required changes in direction of that wavefront.
The optical parameters of an optical component, for example the focal length of a lens, is normally fixed by the characteristics of the material forming the component, together with the shape of the component. In some applications it would, however, be useful to have the ability to change said optical parameters of an optical component by application of electrical signals to the component. In recent years a number of optical components incorporating liquid crystals have been developed in order to achieve electrically controllable optical parameters. In Applied Optics, volume 23, pages 2774-2777 published August 1984 there is described a spherical lens comprising a cascade of two liquid crystal cells each comprising a thin film of nematic liquid crystal confined between two thin glass plates each carrying a transparent electrode. By application of electric fields across the films, a spatial variation in refractive index across each film may be achieved. As, however, the maximum change in optical path length across each film is dependent on the thickness of the film and this thickness is in turn limited by the required transmission of incoming light through the cells and the required response time, only components of very low optical power are possible using this approach.
An attempt to overcome this problem is described in Japanese Journal of Applied Physics, volume 24, pages L626-L628, published August 1985. This article describes a variable-focus lens in which a concave Fresnel lens is used as one wall of a liquid crystal cell, such that the variable-focus lens exhibits the composite focal length of the lens constituted by the liquid crystal and the Fresnel lens. Such a variable-focus lens suffers the disadvantage however that the coarse serrations of the Fresnel lens cause the individual zones of the liquid crystal cell to be too wide and too deep, giving poor focus adjustment and a slow response time, typically over three seconds. Furthermore, the focussing power of such a variable-focus lens cannot be reduced to zero.